The present invention generally relates to an automatic implantable atrial defibrillator for delivering cardioverting or defibrillating electrical energy to the atria of a human heart. The present invention is more particularly directed to a pulse generator for use in an automatic implantable atrial defibrillator which provides the cardioverting or defibrillating electrical energy while exhibiting reduced power consumption of a depletable power source, such as a battery, within the atrial defibrillator.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistent to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has been the cause of these defibrillators from becoming a commercial reality. Two such defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
An implantable defibrillator must be powered by a portable, depletable power source, such as a battery. It has been long believed that as much electrical energy is required to cardiovert or defibrillate the atria of the heart as is required to cardiovert or defibrillate the ventricles of the heart, on the order of ten joules or more. In addition, episodes of atrial fibrillation occur much more frequently than do episodes of ventricular fibrillation. As a result, due to the contemplated required cardioverting or defibrillating energy levels for cardioverting or defibrillating the atria of the heart and the predicted required frequency of delivering such energies, it has long been believed that an implantable atrial defibrillator would deplete its power source so rapidly that frequent battery replacement would be required. Since battery replacement would require the surgical explanting of the defibrillator, it has long been believed that an implantable atrial defibrillator could not be a commercial reality. To this day, a commercially implantable atrial defibrillator remains unavailable.
Defibrillators generally include a means, such as a storage capacitor, for storing the electrical energy required to cardiovert or defibrillate the heart. Since ventricular fibrillation is life threatening, a ventricular defibrillator must charge its storage capacity quickly to permit essentially immediate cardioversion. Such quick storage capacitor charging places an extreme drain on a defibrillator battery thereby limiting the number of times that an implantable ventricular defibrillator can deliver cardioverting or defibrillating energy. This however does not impact upon the commercial nature of such defibrillators because ventricular fibrillation is life threatening and occurs rather infrequently. However, such charging methods believed necessary for an implantable atrial defibrillator has further added to the heretofore non-commercial nature of these devices.
In summary, the previously, believed requirements of quick storage capacitor charging, frequent storage capacitor charging, and high cardioverting defibrillating energy levels attributed to implantable atrial defibrillators has resulted in little or no development by others of a commercially feasible implantable atrial defibrillator. Hence, there remains a need in the art for a commercially viable atrial defibrillator.
The pulse generator of the present invention for use in an implantable atrial defibrillator represents a significant advancement towards a commercially viable implantable atrial defibrillator. The pulse generator of the present invention conserves battery power while still providing adequate electrical energy to cardiovert or defibrillate the atria of the heart to arrest atrial fibrillation. The pulse generator of the present invention achieves this end through the recognition that unlike ventricular fibrillation, atrial fibrillation is not life threatening. Hence, the pulse generator of the present invention charges its storage capacitor comparatively slowly to minimize drain on the defibrillator battery but in sufficient time to arrest the atria fibrillation. This is accomplished by converting the rather low voltage of about three volts provided by the battery to a low duty cycle pulsating high voltage of 300 to 400 volts, for example, for charging the storage capacitor. As disclosed herein in connection with the preferred embodiment, the pulsating high voltage is provided by a flyback transformer coupled to an oscillator which provides a high frequency, low duty cycle output. By virtue of this arrangement, sufficient electrical energy for cardioverting or defibrillating the heart is stored in the storage capacitor without imposing the high drain on the defibrillator battery previously caused by prior art quick charging methods. Even though a minute may be required to fully charge the storage capacity, this is sufficient time to arrest the atrial fibrillation and bring comfort to the patient. The end result is an implantable atrial defibrillator which is commercially viable since it is capable of providing a substantially increased number of cardioverting or defibrillating deliveries of electrical energy before replacement of the defibrillator battery is required.
In addition to the foregoing, the pulse generator of the present invention preferably includes a crosspoint switch for delivering the cardioverting or defibrillating electrical energy from the storage capacitor with a biphasic waveform. Such a biphasic waveform is believed to reduce energy requirements for cardioverting or defibrillating the atria of the heart to further reduce power consumption of the defibrillator battery. Other features and advantages of the present invention shall become apparent hereinafter.